1. Field of the Invention
The present invention relates to a single-resistor-foil pressure sensitive coordinate input system, particularly to a pressure sensitive coordinate input system with a single foil of resistant material and a transistor circuit.
2. Description of Related Art
Conventional resistor pressure sensitive input systems, as shown in FIG. 1, comprise an upper resistor foil X, a lower resistor foil Y and a control circuit. A pair of electrodes X+, X- are connected to the upper resistor foil X on the X-axis. A pair of electrodes Y+, Y- are connected to the lower resistor foil Y on the Y-axis. A certain voltage is applied to the electrodes of both foils. When the surfaces of the foils are pressed on, on an edge without electrodes the voltage is measured, from which the coordinates of the pressure point is derivable.
The control circuit has four terminals X+, X-, Y-, Y- for controlling the conducting behavior of the transistors Q1-Q4. The electrodes X+ and X- on the upper resistor foil X are connected to the transistors Q1 and Q2, respectively. The electrodes Y+ and Y- on the lower resistor foil Y are connected to the transistors Q3 and Q4, respectively. The electrodes X+ and Y+ are via the transistors Q1, Q3 respectively connected to a voltage Vcc. To perform a coordinate measurement on the area of the resistor foils a pressure point on the foils is pressed on. For measuring the X coordinate, the transistor Q1 opens, and a positive voltage is fed to the electrode X+. The transistor Q2 opens, too, and the electrode X- is grounded. The transistors Q3, Q4, connected to the electrodes Y+, Y- of the lower resistor foil Y, block, and no current flows through the lower resistor foil Y. By pressing on the foils, the upper and lower resistor foils X, Y are connected, and from the resulting voltage at AD-X the X coordinate of the pressure point is derived.
For measuring the Y coordinate, the transistor Q3 opens, and a positive voltage is fed to the electrode Y+. The transistor Q4 opens, too, and the electrode Y- is grounded. The transistors Q1, Q2, connected to the electrodes X+, X- of the upper resistor foil X, block, and no current flows through the upper resistor foil X. By pressing on the foils, the upper and lower resistor foils X, Y are connected, and from the resulting voltage at AD-Y the Y coordinate of the pressure point is derived.
Since in this arrangement the voltage Vcc is permanently switched, the voltages VX at the electrode X- and VY at the electrode Y- are not stable. Furthermore, the voltages VX and VY are transmitted via the upper and lower resistor foils X, Y, with a linear dependence on the distance of the pressure point PX, PY to the electrode X-, Y-, and the supply voltage Vcc is permanently switched between the upper and lower resistor foils, therefore both resistor foils have to be made of the same, uniform material, which is difficult to achieve. Noise and unstable voltage easily occur.